This invention relates to a method of manufacturing a knitted garment on a double-cylinder circular knitting machine with automatic needle latch opening, i.e. having the latches of the upper and lower cylinder needles automatically opened before the needles catch the yarn, and to a machine implementing such a method.
As is known, provision is made in a double-cylinder circular knitting machine, within the cam box associated with the upper needle cylinder, for a set of cams which are arranged at each yarn or thread feed to form the stitch and are adjustable in height to adjust the length of the knitted loop formed on the upper cylinder needles. More specifically, these are superimposed cams, movable together in pairs, and defining, in the direction of movement of the upper cylinder sliders, a first raising portion corresponding to the return movement of the needles that have picked up a yarn at a respective feed, a successive partial lowering portion, corresponding to a lowering movement of the needles which have picked up a yarn, said movement having the purpose of bringing the loops, retained by the needles, to the height level of the sinkers adjacent the lower cylinder, and a further short raising portion to bring the sliders of the upper cylinder in condition for selection in order to drive or not the associated needles for forming a stitch at the successive or following feed.
When the partial lowering cam or additional knitting cam is brought to the upper limit allowed by its mobility, and this in order to obtain long stitches, it may happen that the needle is lowered to an insufficient extent for bringing the loop up to the height level of the sinker hook and that the loop stays between the needle hook and the closed latch, or that, while the needle is lowered to a sufficient extent, the loop stays on the same in a position too high for being hooked by the sinker hook at the subsequent withdrawal or return phase of the sinker. This event occurs mainly when there are two or more adjacent needles operating in the upper cylinder and when long stitches are to be formed, e.g. in manufacturing garments such as so-called panty hose, wherefrom a considerable stretching capacity is expected, or when the yarn being worked is thick and/or scarcely extendible.
Therefore, when the loop is not hooked by the hook of the withdrawing sinker, as driven by specially provided cams located in the sinker supporting ring of the lower cylinder, then the yarn is left free and the upper cylinder needles adjacent to said sinker miss the loop. However, it also happens that when, at the following feed, the needle is again lowered to pick up the yarn and the sinker is again extended outwardly, again by the sinker driving or actuating cams, to effect the hooking up of the fresh loop that the needle is going to form, the sinker meets with its back the length of yarn previously not hooked, which has by now been further lowered owing to the further lowering movement of the needle, and pushes it outwards with respect to the cylinder. The yarn, thus pushed, also entrains outwards the adjacent needle, which is thus caused to bend and insert between the latchguard cam and cylinder, thereby damaging the cams and most often a certain number of adjacent needles as well. It then becomes necessary to stop the machine, withdraw the damaged needles and replace them, overhaul the cams and possibly replace them, discharge the knitted work incomplete, before it becomes possible to resume the work. All this involves a long break in the manufacturing process, with considerable loss of productivity. In some cases, it is not uncommon for the upper cylinder knocking-over bits to break, so that it becomes necessary to disassemble the entire upper portion of the machine before it is possible to resume production. Sometimes, moreover, it may happen that the yarn of the loop, stretched between two sinkers, acts on the needle operating in the lower cylinder and dislodges it from its groove so that the overlying slider of the upper cylinder can no longer open the latch of this needle, and the latter can no longer form a stitch.
Since such drawbacks occur the more frequently the thicker is the yarn employed on a given machine, it follows that, for a given machine, it becomes impossible to go to extremes for what concern the thickness of the yarns without undergoing the risk of the failures mentioned above.
In addition, the conformation of the cams just described makes it necessary for the articles, e.g. a stocking article, to be started with the lower cylinder needles, leaving the upper cylinder needles floating, that is allowing these needles to pass high above in order to retain the initial stitch without forming any further stitches. In fact, it would be impossible to operate the needles of the upper cylinder, as it would be preferable instead both for the aesthetics of the final product (which would exhibit wide or crimped stitches on the inside rather than on the outside, as occurring heretofore) and for a reduced wear and stress of the latches of the upper cylinder needles, which keep hitting continuously the latchguard cams, because the lowering of the needles under the action of the additional stitch cam to bring the yarn under the hook of the sinker would cause the needles of the upper cylinder to interfere with the elastic thread which is weft inserted at the start of the stocking article, moving the elastic thread away from the weft position and preventing the weft insertion thereof.
On the other hand, the removal or elimination of the additional stitch cam is not feasible, but for a few very special situations, e.g. when working alternately with a needle operating in the lower cylinder and a needle operating in the upper cylinder; for the great majority of the works, the presence of the additional stitch cam is indispensable, thereby the aforementioned drawbacks are brought about.